The present invention relates to polyimide precursors, to a method for converting these precursors into higher molecular weight polyamic acids and/or polyimidebenzoxazole ("PIBO") polymers, and to films and coatings of PIBO.
In the preparation of polyimidebenzoxazole, a dianhydride and diaminobenzoxazole are first reacted to form a polyamic acid. The polyamic acid is then converted to PIBO by closing the amic acid linkages to form imide rings. In general, the polyamic acids are prepared as a solution in an organic solvent such as N,N'-dimethylacetamide or the like. Heretofore, only a limited number of dianhydride/diaminobenzoxazole combinations have been disclosed as being useful for producing PIBO.
U.S. Pat. No. 4,087,409 to Preston broadly teaches reacting two symmetrical monomers to produce an essentially linear heterocyclic polymer having at least two different heterocyclic linkages. Polyamic acids can be prepared by selecting monomers from an otherwise long list of monomers (e.g. 2,2'-p-phenylene bis(5-aminobenzoxazole) and pyromellitic dianhydride). See also, the articles "Fibers from Ordered Benzheterocycle-Imide Copolymers," Appl. Poly. Sym., No. 9, pp. 145-158 (1969) and "New High-Temperature Polymers, VIII. Ordered Benzoxazole- and Benzothiazole-Imide Copolymers," J. Poly. Sci., Part A-1 (1969), 7(1), pp. 283-296.
"Azole Analogs of Polypyromellitimides,"Vysokomol. soed., Vol. (A) XIII, No. 11, 1971, pp. 2565-2570, discloses synthesis of azole containing analogs of poly-[N,N'-(p,p'-phenoxyphenylene)pyro-mellitimide]. A PIBO prepared from 2,6-di(p, p'-aminophenoxyphenyl)benzo[1,2-d:5,4-d']bisoxazole and pyromellitic dianhydride is described.
U.S. Pat. No. 4,866,873 to Mukai et al., discloses an aromatic heterocyclic polyimide comprising substantially equimolar amounts of a specific aromatic, trans-benzobisoxazole or trans-benzobisthiazole diamine such as 2,6-(4,4'-diaminodiphenyl)benzo[1,2-d:4,5-d']bisoxazole and a specific aromatic tetracarboxylic dianhydride such as pyromellitic dianhydride. These polyamic acid precursors are prepared in an amide solvent and converted to PIBO. Similar PIBO polymers prepared from a polyamic acid solution are taught in Japanese Pat. Appl. No. 2-41819 to Mitsubishi Kasei Corporation and in "Novel Aromatic Heterocyclic Polyimide (PIBT) Having Ultra-High Modulus of Elasticity" by Seiichi Nozawa, Kagaku to Kogyo, 44(7), 1154 (1991).
"Synthesis and Mechanical Properties of Novel Polyimide Containing Heterocycles" by Nozawa, Taytama, Kimura and Mukai presented at the 22nd International SAMPE Technical Conference (held Nov. 6-8, 1990), teaches synthesis of one type of trans-PIBT or trans-PIBO using a polyamic acid precursor of 4,4'-diamino-phenylenebenzobisthiazole and pyromellitic dianhydride. Japanese Patent Application No. 41-42458 (Patent KOHO No. 45-8435) to Taoka Senryo Seizo K. K. teaches the synthesis of PIBO from a polyamic acid with the polyamic acid being prepared from an aromatic diamine containing a single benzoxazole ring such as 5-amino-2-(4-aminophenyl)benzoxazole and an aromatic tetracarboxylic acid dianhydride using thermal ring-closure techniques.
While some of the described PIBO polymers have been formed into fibers and films, they do not generally have sufficient physical and/or chemical properties (such as tensile modulus, tensile strength, elongation-to-break and coefficient of thermal expansion (CTE)) to make them generally useful in applications such as for the electronics industry.
In addition, polyamic acids used in preparing PIBO polymers can form highly viscous solutions at relatively low solids content. For example, solutions of polyamic acid containing as little as 7 weight percent of a high molecular weight polyamic acid can exhibit a viscosity of 100,000 centipoise (cps) or more. For various applications such as casting of films using conventional techniques, these high viscosities can be acceptable; for other applications such as spin-casting of coatings, lower viscosities are required or desired.
European Patent Publication No. 0 355 927 to Asahi discloses a photopolymerizable polyimide precursor comprising a specific polyamic acid ester, polyamic amide or polyamic acid salt structure derived from a tetracarboxylic acid compound, a specific diamine compound and an alcohol such as 2-hydroxyethyl acrylate or an epoxy compound such as glycidyl methacrylate. Each of the polymer repeat units of the described polymer contains a pendant organic group having 1 to 20 carbon atoms. The reference also suggests that when a group such as --C.tbd.N or --C.tbd.CH or a maleic anhydride residue is introduced into the precursor as a terminal group, the molecular weight of the ultimate polyimide produced by heat-curing can be increased. The polymers form a composition photopolymerizable with a photopolymerization initiator. The resulting PIBO polymer can be used to coat substrates for subsequent use in photolithography. Films produced from these cross-linked polymers exhibit relatively low tensile strengths and tensile modulus.
In view of the deficiencies in the prior art, it remains desirable to prepare polyimide precursor solutions from which PIBO polymers having a desired combination of chemical and physical properties can be prepared, including, when desired, a polyimide precursor solution having a higher solids content at acceptable viscosities for film forming applications, e.g., spin-casting. In addition, it remains desirable to convert these PIBO polymers into useful films and coatings.